More particularly the invention relates to a method of generating at least one image of a substrate from radioactive radiation coming from a number I at least equal to 2 radioactive sources, these radioactive sources comprising at least one radioactive tracer contained in the substrate, this method comprising the following stages:
a) the radioactive radiation emitted by the substrate is detected in a certain observation field sub-divided into K.times.L pixels, by means of a detector which generates for each pixel (x.sub.k,y.sub.1) of the observation field a detection signal f (x.sub.k,y.sub.1) representing the detection or non-detection of a radioactive emission in this pixel, PA1 b) and the data representing the different detection signals generated by the detector are memorised, individually for each radioactive emission detected during a determined period called the observation period, these memorised data corresponding at least to an estimated position of an emission point of radioactive radiation in the substrate. PA1 c) from the set of memorised data, determining a distribution H(x.sub.k,y.sub.1, A1.sub.j1, . . . , Aq.sub.jq) of the detections corresponding to these data, corresponding to the number of detections at each estimated emission point of coordinates (x.sub.k, y.sub.1) with each combination A1.sub.j1, . . . , Aq.sub.jq of the characteristics of the different criteria, PA1 d) and estimating a distribution h'i(xk,yl) of detections corresponding to each radioactive source i, which minimises the respective deviations between the distribution H(x.sub.k, y.sub.1, A.sub.j1, . . . ,A.sub.jq) and the corresponding values: ##EQU1## PA1 i being an integer between 1 and I, and referring to the radioactive source being considered, PA1 j1 being an integer between 1 and J1, referring to the j1st characteristic possible for the criterion no. 1, PA1 . . . PA1 jq being an integer between 1 and Jq referring to the jq.sup.th characteristic possible for the criterion no. q, PA1 m being an index between 1 and L, PA1 n being an index between 1 and K, PA1 the radioactive sources comprise a plurality of tracers, to which as many distributions h'i(xk,yl) correspond, each forming the image of the distribution of one of said tracers in the substrate; PA1 one of the radioactive sources is a background radiation; PA1 at least one of the radioactive sources is an imaginary source corresponding to at least one defect of the detector; PA1 the detector is a gas detector in which the gamma radiation generates avalanches of electrons on the basis of the detection signal, and said detector comprising hot spots constituted by spots in which are generated avalanches of electrons without detection of beta radiation, these hot spots constituting the imaginary source mentioned above; PA1 each distribution h'i(xk,yl) is estimated by minimising an error function Er defined by: ##EQU2## PA1 the indices .lambda..sub.j1 . . . .lambda..sub.jq are all equal to 1; PA1 at least one of the criteria is chosen from the group consisting in: PA1 the estimated emission point of each detection is the barycentre of the pixels affected by this detection, of co-ordinates: ##EQU3## PA1 the impulse responses Ri,j1, . . . to jq(x,y) taken into account are zero except when (x,y)=(0,0).
The use has already been suggested of a method of this type for generating, using an apparatus for imaging with a solid scintillator (marketed under the trade name ".mu.-imager" by the company BIOSPACE MESURES, Paris, France), two images corresponding respectively to the distribution in the substrate of two different tracers (.sup.35 S and .sup.32 P) emitting radiation with different energy spectra; by using separation criteria of these emissions (such as the number of pixels affected by a detection, the maximum amplitude of the detection signal for a detection, the sum of the amplitudes of the detection signals for the same detection, etc.), each detection is classified more or less in real time as corresponding either to one tracer, or to the other (see the thesis of P. Laniece: "Quantization in hybridization in situ and in autoradiography: development of a high resolution radio-imager", Institut de Physique Nucleaire, 1992 and Y. Charon: "Radio-imaging and biology", thesis of the UniversiteDenis Diderot,--Paris VII, 1995). In this case, the emission point of each radiation detected is likewise estimated, more or less in real time, at the same time as this radiation is attributed to one or the other of the tracers, and each of the images finally generated consists simply in the superposition of the estimated emission points of the different radiation detected, respectively for each tracer.